1. Field of the Invention
The present invention relates to a color image forming apparatus such as a color printer or a color copier.
2. Description of the Related Art
In recent years, a color image forming apparatus such as a color printer or a color copier which employs an electrophotographic method, an ink-jet method or the like, is required to improve the image quality of an output image. The density gradation and stability thereof, in particular, greatly influence a user's judgment as to whether an image is good or bad.
However, if the constituent elements of the color image forming apparatus change according to an environmental change or due to use over a long time, the density of the output image changes. In a color image forming apparatus which employs an electrophotographic method, in particular, density changes and color balance is disturbed even if the environment only slightly changes. Due to this, it is necessary to provide means for always maintaining image density constant
To this end, the color image forming apparatus is constituted to obtain stable images by forming a density detection toner image (to be referred to as a “patch” hereinafter) on an intermediate transferring body, a photosensitive member or the like, detecting the density of an unfixed toner patch using an unfixed toner density detection sensor (to be referred to as “density sensor” hereinafter), feeding back process conditions including exposure quantity and developing bias based on the detection result, and thereby controlling the image density.
The density control using the density sensor is, however, intended to detect image density by forming a patch on an intermediate transferring body, drum or the like and not to control a change in the color balance of the image caused by the transfer and fixing of the patch onto the transferring material. Thus, the density control using the density sensor cannot deal with this latter change.
To deal with this change, therefore, a color image forming apparatus provided with a sensor which detects the density or color of a patch on a transferring material (which sensor will be referred to as a “color sensor” hereinafter) is considered.
This color sensor is constituted so that three or more types of light sources such as red (R), green (G) and blue (B) different in emission spectrum are used as light emitting elements, or a light source which emits white color (W) is used as a light emitting element, and three or more types of filters, such as red (R), green (G) and blue (B) filters, different in spectral transmittance are formed on the light receiving elements. By so constituting, it is possible to obtain three or more different types of outputs such as RGB outputs.
To control color density using the color sensor, however, it is required to form a patch on a transferring material, thus consuming the transferring material and toners. It is, therefore, impossible greatly to increase the frequency with which control is performed. Accordingly, it is necessary effectively to control density while minimizing control executing frequency using the color sensor.